The present invention is generally related to circuits that sense differential signals with high common-mode voltage. More particularly, the present invention is related to an apparatus for sensing differential voltages with high common-mode levels such that noise, offset, and drift effects in the resulting circuit are minimized.
A schematic diagram of a conventional dynamic bridge amplifier (100) is illustrated in FIG. 1. The dynamic bridge amplifier includes five resistors (R10-R14) and an operational amplifier (A1).
Resistor R1 is connected between V1 and node N1. Resistor R1xe2x80x2 is connected between V2 and node N2. Resistor R2 is connected between node N1 and GND. Resistor R2xe2x80x2 is connected between node N2 and GND. Resistor R3 is connected between node N2 and VO. Operational amplifier A1 includes a non-inverting input terminal that is connected to node N1, an inverting input terminal that is connected to node N2, and an output that is connected to VO.
In operation, an input signal is applied across the V1 and V2 terminals, and an output signal is provided at the VO terminal. The signal includes a common-mode portion (VCM) and a differential portion (VDIFF). The differential portion is a small signal, while the common-mode portion may be rather large. The signal levels are given by: V1=VCMxe2x88x92VDIFF/2 and V2=VCM+VDIFF/2.
R1 is large compared to R2 such that the common-mode voltages that are seen by the inputs of the operational amplifier (A1) are reduced. Amplifier A1 drives a signal into the VO terminal until the voltage at node N1 and N2 are equal. The transfer function that is associated with circuit 100 can be described by the following equation: VO/VDIFF=xe2x88x92(1/2R1)*{R3[1+R2/R2xe2x80x2]+R2}. When resistors R3 and R2 have an equivalent parallel resistance as given by R3//R2xe2x80x2=R2, the transfer function may be simplified as: VO/VDIFF=xe2x88x92R3/R1.
Briefly stated, the present invention is related to a differential voltage amplifier that includes a dynamic level shifter circuit and an amplifier circuit. The dynamic level shifter circuit includes high-impedance current sources and resistors that move the common-mode levels of a differential input signal to a signal level that is suitable for the amplifier circuit. The amplifier circuit may be single-ended or differential. The dynamic level shifter circuit may include one or more additional current sources that are arranged to provide improved performance for low common-mode levels. A dynamic biasing scheme may be employed to improve operation over varied common-mode ranges. A trimming circuit may be used to adjust offsets in the system. A DC chop arrangement may be employed to remove offsets in the system.
A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are briefly summarized below, the following detail description of presently preferred embodiments of the invention, and the appended claims.